Move Capstanfile parsing to frontend

Change the Build() function signature to take a Template and move
`Capstanfile` parsing to frontend. This allows "capstan run" to build a
template on the fly to support running Jar files directly.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Fix broken build

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>